A Digital Television Standard published Sept. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 Hz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238 * 10.sup.6 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) for terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value+S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511 -sample pseudo-random noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle one of these 63-sample PN sequences is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. The other two 63-sample PN sequences and the 511 -sample PN sequence are transmitted in accordance with the same logic convention in all data fields.
The data within data lines are trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit. The interleaved trellis codes have been subjected to Reed-Solomon forward error-correction coding, which provides for correction of burst errors arising from noise sources such as a nearby unshielded automobile ignition system. The Reed-Solomon coding results are transmitted as 8-level (3 bits/symbol) one-dimensional-constellation symbol coding for over-the-air transmission, which transmissions are made without symbol preceding separate from the trellis coding procedure. The Reed-Solomon coding results are transmitted as 16-level (4 bits/symbol) one-dimensional-constellation symbol coding for cablecast, which transmissions are made without trellis coding. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed.
The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
VSB signals using 8-level symbol initilly be used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding can be used in over-the-air narrowcasting systems or in cable-casting systems. However, certain cable-casting is likely to be done using suppressed-carrier quadrature amplitude modulation (QAM) signals instead, rather than using VSB signals. This presents television receiver designers with the challenge of designing receivers that are capable of receiving either type of transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received.
It is assumed that the data format supplied for symbol encoding is the same in transmitters for the VSB DTV signals and in transmitters for the QAM DTV signals. The VSB DTV signals modulate the amplitude of only one phase of the carrier at symbol rate of 10.76 * 10.sup.6 symbols per second to provide a real signal unaccompanied by an imaginary signal, which real signal fits within a 6 MHz band because of its VSB nature with carrier near edge of band. Accordingly, the QAM DTV signals, which modulate two orthogonal phases of the carrier to provide a complex signal comprising a real signal and an imaginary signal as components thereof, are designed to have a symbol rate of 5.38 * 10.sup.6 symbols per second, which complex signal fits within a 6 MHz band because of its QAM nature with carrier at middle of band.
Processing after symbol decoding is similar in receivers for the VSB DTV signals and in receivers for the QAM DTV signals, assuming the data format supplied for symbol encoding is the same in transmitters for the VSB DTV signals and in ransmitters for the QAM DTV signals. The data recovered by symbol decoding are supplied as input signal to a data de-interleaver, and the de-interleaved data are supplied to a Reed-Solomon decoder. Error-corrected data are supplied to a data de-randomizer which regenerates packets of data for a packet decoder. Selected packets are used to reproduce the audio portions of the DTV program, and other selected packets are used to reproduce the video portions of the DTV program.
The zero-intermediate-frequency (ZIF) receivers, which perform amplification and channel selection at baseband, that are used for receiving QAM DTV signals are not well suited for receiving VSB DTV signals. This is because of problems with securing adequate adjacent-channel rejection in a ZIF receiver when the carrier is not at the center frequency of the channel. The tuners can be quite similar in receivers for the VSB DTV signals and in receivers for the QAM DTV signals if the receivers are of superheterodyne types, however. The differences in the receivers reside in the synchrodyning procedures used to translate the final IF signal to baseband and in the symbol decoding procedures. A receiver that is capable of receiving either VSB or QAM DTV signals is more economical in design if it does not duplicate the similar tuner circuitry prior to synchrodyning to baseband and the similar receiver elements used after the symbol decoding circuitry. The challenge is in optimally constructing the circuitry for synchrodyning to baseband and for symbol decoding to accommodate both DTV transmission standards and in arranging for the automatic selection of the appropriate mode of reception for the DTV transmission currently being received.
Radio receivers for receiving VSB DTV signals, in which receiver the third mixer output signal is a final intermediate-frequency signal somewhere in the 1-8MHz frequency range rather than at baseband, are described by the inventors in the U.S. patents listed below and incorporated by reference herein:
U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 to C. B. Patel and A. L. R. Limberg, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER.
U.S. Pat. No. 5,548,617 issued Aug. 20, 1996 to C. B. Patel and A. L. R. Limberg, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVER.
U.S. Pat. No. 5,606,579 issued Feb. 25, 1997 for C. B. Patel and A. L. R. Limberg, entitled DIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVER.
U.S. Pat. No. 5,659,372 issued Aug. 19, 1997 to C. B. Patel and A. L. R. Limberg, entitled DIGITAL TV DETECTOR RESPONDING TO FINAL-IF SIGNAL WITH VESTIGIAL SIDEBAND BELOW FULL SIDEBAND IN FREQUENCY.
U.S. Pat. No. 5,715,012 issued Feb. 3, 1998 to C. B. Patel and A. L. R. Limberg, entitled RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS.
U.S. Pat. No. 5,731,848 issued Mar. 24, 1998 to C. B. Patel and A. L. R. Limberg, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING NG FILTERS, AS FOR USE IN AN HDTV RECEIVER. The final IF signal is digitized and the synchrodyne procedures are carried out in the digital regime. In radio receivers that are to have the capability of receiving DTV signals no matter whether they are transmitted using VSB or QAM, conversion of the signals to final. IF signals just above baseband permits the frequency of the oscillations of the third local oscillator to remain substantially the same no matter whether VSB or QAM transmissions are being received. The differences in carrier frequency location within the channel can be accommodated in the synchrodyning procedures carried out in the digital regime.